Getting pointcloud colors from GLB file

Hello,

The `.glb` file produced by the code does not seem to contain the vertex colors for the pointcloud, but the documentation seems to indicate otherwise. Am I looking at the right place?

```python
>>> x = trimesh.load("scene.glb")
>>> x.geometry["geometry_0"]

>>> x.geometry["geometry_0"].visual.vertex_colors
TrackedArray([], dtype=float64) # Empty?
```

Thanks!